Frequency synthesizers adaptive loop filter with compensation for transients

ABSTRACT

An improved adaptive loop filter for modulatable frequency synthesizers containing digital phase comparators is described. The unique adaptive loop filter provides a wide bandwidth during a signal acquisition mode and a narrow bandwidth during a signal tracking mode. The adaptive loop filter includes a current limiter coupled to the error signal from the digital phase comparator for generating a current-limited output signal; a filter coupled by a resistor to the current limiter output signal and including a series-coupled resistor and capacitor for filtering the current limiter output signal to provide a steering line voltage; an amplifier coupled to the junction between the filter resistor and capacitor for generating a feedback signal; a resistor for coupling the feedback signal to the current limiter output signal for substantially reducing the duration of voltage transients; and a reference filter for coupling the steering line voltage to the steering input of a voltage-controlled oscillator. The wide acquisition loop bandwidth and the narrow tracking loop bandwidth are provided by switchably coupling different resistors to the filter capacitor. The phase shift produced by the reference filter is also changed depending on whether the acquisition or steering mode is active.

BACKGROUND OF THE INVENTION

The present invention relates generally to frequency synthesizers, andmore particularly to an improved dual-bandwidth loop filter for use infrequency synthesizers of the type containing digital phase comparators.

In radio frequency synthesizers, it is desirable to have fast frequencylocking characteristics when switching to a desired radio signalfrequency, while at the same time, adequately attenuating referencesignal feedthru and modulating the synthesizer output with signalshaving frequencies as low as 200 Hz once the synthesizer has locked ontothe desired radio signal frequency. In order to provide fast frequencylocking characteristics, prior art synthesizers, such as the describedin U.S. Pat. No. 4,330,758, provide a wide loop bandwidth in order tolock quickly to a desired radio signal frequency and a narrower loopbandwidth once frequency lock has been obtained in order to attenuatereference signal feedthru. However, in switching from a wide to a narrowloop bandwidth, such prior art synthesizers typically introduce voltagetransients which result in undesirable noise on the synthesizer outputsignal, the duration of which is stretched and emphasized due to thenarrower loop bandwidth. Furthermore, much more costly and complexcircuitry is required in the sample-and-hold phase comparator used inthe synthesizer in U.S. Pat. No. 4,330,758 than in a digital phasecomparator. In prior art synthesizers containing digital phasecomparators, an extended modulation signal bandwidth has been obtainedby modulating both the reference oscillator and the voltage-controlledoscillator. However, this scheme not only requires an expensivereference oscillator, but also results in excessive reference signalfeedthru.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved adaptive loop filter for frequency synthesizers that greatlyattentuates noise due to bandwidth switching and reference signalfeedthru.

It is another object of the present invention to provide an improvedadaptive loop filter for frequency synthesizers that locks on frequencyquickly and is also modulatable at audio signal frequencies as low as200 Hz.

Briefly described, the present invention encompasses an improved loopfilter for frequency synthesizers of the type including a signal sourcefor generating a reference signal, a phase detector coupled to thereference signal and a first feedback signal for generating an errorsignal, a voltage-controlled oscillator (VCO) coupled to the loopfilter, and a divider coupled to the VCO for providing the firstfeedback signal. The improved loop filter includes a current limitercoupled to the error signal from the phase detector for generating acurrent-limited output signal; a first resistor coupled to the currentlimiter output signal; a filter coupled to the first resistor andincluding a series-coupled second resistor and capacitor for filteringthe current limiter output signal to provide a steering signal which iscoupled to the VCO; an amplifier coupled to the junction between theseries-coupled second resistor and the capacitor for generating a secondfeedback signal; and a third resistor coupling the second feedbacksignal to the current limiter output signal for substantially reducingthe duration of voltage transients on the current limiter output signal.

BRIEF DESCRIPTION OF THE DRAWING

A frequency synthesizer including a detailed circuit diagram of anadaptive loop filter embodying the present invention is illustrated inthe drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, there is illustrated a frequency synthesizeror a radio transmitter that includes an improved adaptive loop filter114, 150 and 140 embodying the present invention. The adaptive loopfilter 114, 150 and 140 is responsive to a bandwidth control signal forswitching the loop bandwidth of the synthesizer between a wide bandwidthfor locking the loop quickly and a narrow bandwidth once lock isobtained. In the preferred embodiment, the loop bandwidth is switchedbetween a wide bandwidth of 600 Hz and a narrow bandwidth of 80 Hz. Thenarrow bandwidth has been selected to be 80 Hz so that the frequencyresponse of the synthesizer will be relatively flat at audio signalfrequencies as low as 200 Hz. As a result, the synthesizer can bemodulated by a modulation signal, such as audio signals or digital datasignals, having frequencies ranging from as low as 200 Hz to 3,000 Hz orhigher.

In addition to loop filter 114, 150 and 140, the synthesizer includes avoltage-controlled oscillator (VCO) 112 having a steering input coupledto the output of the loop filter, a mixer 156 coupled to an externalreference signal and the output signal of VCO 112, a divider 108 coupledto the output of mixer 156, a reference oscillator 102 providing areference signal of a predetermined frequency, a reference divider 106coupled to the reference signal, and a phase detector 110 coupled to theoutputs of reference divider 106 and divider 108. In a radiotransceiver, the external reference signal can be provided by anothersynthesizer in the receiver. In such radio transceivers, mixer 156 isused to maintain a constant offset between the frequencies of the outputsignals from the transmitter synthesizer and receiver synthesizer. Inother applications, the VCO output signal can be connected directly todivider 108.

Reference divider 106, divider 108 and phase detector 110 can beprovided by commercially available frequency synthesizer integratedcircuits 104, such as the Motorola type MC145146 and MC145156 frequencysynthesizer integrated circuits manufactured by Motorola SemiconductorProducts, Inc., Austin, Texas. Specifications describing the operationand circuitry of the MC145146 and MC145156 frequency synthesizer arepublished by an available from Motorola Semiconductor Products, Inc.Separate integrated circuits are also commercially available forreference divider 106, divider 108 and phase detector 110.

The output signal provided by the synthesizer is taken from VCO 112 andmay be coupled to the transmitting circuitry of a conventional radiotransceiver. VCO 112 may also be coupled to a modulation signal, whichmay be an audio signal such as voice signals from a microphone or adigital signal such as a binary data signal. The synthesizer andtransmitting circuitry are described in further detail in Motorolainstruction manual no. 68P81061E10 entitled "MCR 1200 Nordic MobileTelephone", published by and available from the Technical WritingServices Department, Motorola, Inc., 1301 East Algonquin Road,Schaumburg, Illinois.

Adaptive loop filter 114, 150 and 140 includes a current limiter 114,lag filter 150 and reference filter 140. Current limiter 114 includestransistors 116 and 122 which generate a current-limited output signalby bleeding off excess current from the error signal output of phasedetector 110 to the +V supply through resistor 123 or to signal groundthrough resistor 117. This operation is possible since the tri-stateoutput of phase detector 110 in MOS synthesizer integrated circuits 104possesses inherent current limiting characteristics. A relatively smallcurrent through resistor 120 biases either transistor 116 or transistor122 into the on state in response to a high or low pulse, respectively,from the error signal output of phase detector 110. The error signalfrom phase detector 110 is a high or low pulse having a repetition rateequal to the frequency of the reference signal from reference oscillator102 divided by the pre-selected modulus of divider 106.

The synthesizer has two operational states, a 600 Hz bandwidth state oracquisition mode, and an 80 Hz bandwidth state or tracking mode. Duringthe quiescent 80 Hz bandwidth state, the output of phase comparator 110spends most of the time in a high impedance tri-state mode, andgenerates high or low pulses as required to keep the loop in phase lockduring minor perturbations, such as noise generated in the internalcircuitry, or voltage variations due to leakage current throughcapacitors in the loop filter. This is true in general of digital phasecomparators having a tri-state output. These pulses result in necessaryreference signal energy at the phase detector output, which is filteredand greatly attenuated by the loop filter before reaching the steeringinput of VCO 112. Additional unnecessary reference signal energy isgenerated by the residual capacitance in the phase comparator output dueto the fact that the voltage at its output will usually remain near +Vor ground after each pulse since most conventional phase comparators(such as those in the MC145146 and MC145156 synthesizer integratedcircuits) do not switch the pull-up and the pull-down transistors in thetri-state output circuit to the tri-state mode at exactly the same time.In prior art synthesizers, the energy stored in the residual capactanceof the phase detector output is discharged into a typically large valueresistor 124, causing substantial unnecessary reference energy whichappears as undesirable spurious energy on the VCO output signal.

According to an important feature of the present invention, theinclusion of resistor 132 and amplifier 138 discharges the residualcapacitance of the phase detecter output rapidly, thus minimizingunnecessary reference signal energy and substantially eliminatingreference signal feedthru. When the output of phase comparator 110 is inthe tri-state mode, the residual capacitance is discharged by amplifier138 through resistor 132 and current limiter 114. Due to the fact thatthe average voltage on each end of resistor 132 is the same in thequiescent state, the presence of the resistor 132 does not disturb thehigh impedance tri-state mode of the output of phase detector 110. Thisis necessary to maintain proper operation of phase detector 110.

The loop bandwidth of the synthesizer is switched between a widebandwidth of approximately 600 Hz in the acquisition mode and a narrowbandwidth of approximately 80 Hz in the tracking mode by turning on andturning off transmission gates 126 and 128. Although transmission gates126, 128 and 146 are shown functionally as blocks, they are field effecttransistors in the preferred embodiment of the present invention. In thesignal acquisition mode, transmission gates 126 and 128 turn onproviding a wide loop bandwidth when the bandwidth control signal has abinary zero state. When turned on, transmission gate 126 substantiallyshorts out resistor 124. When transmission gate 128 is turned on,current limiter 114 is coupled to capacitor 136 by resistor 130. Thevalue of resistor 130 is preferably chosen to be much smaller than thevalue of resistors 124 and 134. As a result, in the acquisition mode,the amount of current coupled to capacitor 136 during each error pulsefrom phase detector 110 is increased. Therefore, the magnitude of thecontrol voltage signal can be changed more quickly, and in turn thefrequency of VCO 112 is correspondingly changed more quickly in theacquisition mode. For example, current pulses of as much as twomilliamps may be generated for charging and/or discharging capacitor 136when transmission gates 126 and 128 are turned on. The maximum amount ofcurrent is limited only by the current output capacity of phase detector110, and is held constant over varying steering line voltages by currentlimiter 114.

In the tracking mode, transmission gates 126 and 120 turn off providinga narrow loop bandwidth when the bandwidth control signal has a binaryone state. When transmission gates 126 and 128 are turned off, capacitor136 is charged by much smaller current pulses by means of resistors 124and 134. For example, when the desired frequency of the output signalfrom VCO 112 is reached, the bandwidth control signal may be switchedfrom the binary zero state to the binary one state by other circuitry,such as a loop-lock detector (not shown). When the loop is locked, thesteering line voltage stored on capacitor 136 and the frequency of VCO112 is changed in much smaller increments via resistors 124 and 134. Forexample, the current pulses coupled through resistors 124 and 134 tocapacitor 136 may be on the order of fifty microamps or less when theloop is locked.

Resistor 130 and resistors 124 and 134 essentially provide constantcharging and discharging currents to capacitor 136 in response to eachlow or high pulse, respectively, of the error signal output of phasedetector 110. The magnitude of the charging/discharging current islimited by, but not directly determined by, current limiter 114. Thecurrent from current limiter 114 is divided by resistor 132 and eitherresistor 124 or 130 depending upon whether the acquisition or trackingmode is active. In the tracking mode, most of the current goes throughresistor 132 since resistor 124 is greater in value than resistor 132,and produces a voltage drop across resistor 132 which is independent ofthe steering line voltage. In this case, the charrging/dischargingcurrent into capacitor 136 is determined by resistors 124 and 132 andthe voltage drop across resistor 132. This configuration allows resistor124 to be very large in value, without creating unnecessary referencefeedthru. In the acquisition mode, most of the current goes throughresistor 130 since resistor 130 is smaller than resistor 132. Thevoltage drop produced across resistors 132 and 130 is also independentof the steering line voltage. The charging/discharging current intocapacitor 136 is now determined by the resistance of, and voltage dropacross, resistor 130.

In the signal tracking mode, the magnitude of the current pulses appliedto capacitor 136 is primarily determined by resistor 124, assumingresistor 124 is much greater than resistor 134. In this mode, thenatural frequency W_(n) and dampening factor D for the loop can beexpressed as follows:

    W.sub.n =G/C136;

and

    D=(W.sub.n R134 C136)/2;

where G is the open-loop gain, R134 is resistor 134, and C136 iscapacitor 136. The open-loop gain G is a function of resistors 120, 132,124 and 134. In the signal acquisition mode, the magnitude of thecurrent pulses applied to capacitor 136 is primarily determined byresistor 130, assuming resistor 134 is much greater than resistor 130.In this mode, the natural frequency W_(n) and dampening factor D for theloop can be expressed as follows:

    W.sub.n =G/C136;

and

    D=(W.sub.n R130 C136)/2;

where R130 is resistor 130, and C136 is capacitor 136. The open-loopgain G is now a function of resistors 120, 132 and 130.

In order to be able to modulate the synthesizer with signals havingfrequencies as low as 200 Hz, it is necessary that the natural frequencyW_(n) be rather small. The natural frequency W_(n) is made small bymaking the current pulses from phase detector 110 small. Therefore, inthe tracking mode, the current pulses from phase detector 110 are madesmall (e.g. fifty microamps) by making resistor 124 large. Resistor 124can be made very large in value, since most of the current from eachphase detector pulse goes through resistor 132.

Referring to reference filter 140, reference signal feedthru is furtherattenuated by a two-section filter, the first section of which iscomprised of resistor 142 and capacitor 144 and the second section ofwhich is comprised of resistor 148 and capacitor 154. Another feature ofthe present invention is that the phase shift produced by referencefilter 140 is changed when switching bandwidths in response to thebandwidth control signal. Transmission gate 146 is turned on in responseto a binary one state of the bandwidth control signal in the signaltracking mode, and is turned off in response to a binary zero state ofthe bandwidth control signal to provide less phase shift at low signalfrequencies so that the loop will remain stable in the acquisition mode.For example, the phase shift may be reduced from forty-nine degrees forsignals having a frequency of 600 Hz when transmission gate 146 is onand maximum reference signal attenuation is desired in the trackingmode, to a phase shift of twenty-eight degrees for signals having afrequency of 600 Hz when transmission gate 146 is off and loop stabilityis desired in the acquisition mode.

In addition, according to another feature of the present invention,transmission gate 146 is coupled in parallel across resistor 152 toprovide transient free switching. That is, resistor 152 is coupledbetween capacitors 154 and 144 and signal ground for establishing avoltage substantially equal to signal ground at the terminals ofcapacitors 154 and 144 that are coupled to transmission gate 146.Therefore, when transmission gate 146 turns on to switch to the trackingmode, no transients are generated since capacitors 154 and 144 havereached signal ground during the acquisition mode. In contrast,resistors 148 and 142 are shorted out during the acquisition mode insome prior art synthesizers. Thus, when switching to the tracking mode,such prior art synthesizers generate a voltage transient on the steeringline voltage. Switching between the acquisition mode and tracking modeis transient free in reference filter 140 since the voltages acrossresistors 148 and 142 developed due to leakage currents in the steeringinput of VCO 112 will reach quiescent value during the acquisition modeand remain the same when switching to the tracking mode.

In summary, an improved adaptive loop filter for frequency synthesizershas been described that is modulatable over a wide range of audio signalfrequencies, while at the same time obtaining lock quickly when thesynthesizer frequency is changed. The unqiue adaptive loop filter alsoprovides enhanced noise rejection characteristics, so that referencesignal feedthru and switching transients are greatly attentuated. Theadaptive loop filter of the present invention provides a wide loopbandwidth for signal acquisition and a narrow loop bandwidth for signaltracking once frequency lock has been obtained. The unique adaptive loopfiler can be used in any suitable frequency synthesizer, such as thosetypically employed in the transmitter and receiver portions of radiotransceivers.

I claim:
 1. A loop filter for frequency synthesizing means, saidfrequency synthesizing means including a signal source for generating areference signal, phase detecting means coupled to the reference signaland to a first feedback signal for generating an error signal,voltage-controlled oscillating (VCO) means coupled to the loop filter,and dividing means coupled to the VCO means for providing the firstfeedback signal, said loop filter comprising:current limiting meanscoupled to the error signal from the phase detecting means forgenerating a current-limited output signal; first resistive meanscoupled to receive the current limiting means output signal; filteringmeans coupled to receive the output of the first resistive means andincluding a series-coupled second resistive means and capacitive means,for filtering the current limiting means output signal to provide asteering signal, the steering signal being coupled to the VCO means;amplifying means coupled to receive the output of the capacitive meansfor generating a second feedback signal; and third resistive meanscoupling the second feedback signal to the current limiting means outputsignal for substantially reducing the duration of transients of thecurrent limiting means output signal.
 2. The loop filter according toclaim 1, further including second filtering means intercoupled betweenthe first filtering means and the VCO means for filtering the steeringsignal.
 3. The loop filter according to claim 1, wherein the capacitivemeans of the filtering means is coupled to signal ground and the secondresistive means is coupled between the steering signal and thecapacitive means, said loop filter further including fourth resistivemeans and first and second switching means, the first switching meansbeing coupled in parallel with the first resistive means, and the secondswitching means and fourth resistive means being coupled in seriesbetween the current limiting means and the capacitive means, said firstand second switching means switching on in response to a first state ofa bandwidth control signal and switching off in response to a secondstate of the bandwidth control signal for changing the bandwidth of saidloop filter.
 4. The loop filter according to claim 3, further includingsecond filtering means intercoupled between the first filtering meansand the VCO means for filtering the steering signal.
 5. The loop filteraccording to claim 4, wherein said second filtering means produces aswitchable phase shift and includes third switching means, said thirdswitching means switching off in response to the first state of thebandwidth control signal and switching on in response to the secondstate of the bandwidth control signal for changing the phase shiftprovided by said second filtering means.
 6. The loop filter according toclaim 4, wherein said second filtering means includes third switchingmeans and first and second phase shifting means, said first phaseshifting means including a fifth resistive means and a second capacitivemeans, and said second phase shifting means including a sixth resistivemeans and a third capacitive means, said fifth and sixth resistive meanscoupled in series between the steering signal and the VCO means, saidsecond capacitive means being coupled between a seventh resistive meansand intercoupled fifth and sixth resistive means, the seventh resistivemeans further being coupled to signal ground, the third capacitive meansbeing coupled between the VCO means and the seventh resistive means, andthe third switching means being coupled in parallel with the seventhresistive means, said third switching means switching off in response tothe first state of the bandwidth control signal and switching on inresponse to the second state of the bandwidth control signal forchanging the phase shift provided by said first and second phaseshifting means.
 7. The loop filter according to claim 2, wherein saidsecond filtering means comprises first and second phase shifting meanscoupled in series between the first filtering means and the VCO meansfor filtering and phase shifting the steering signal.
 8. The loop filteraccording to claim 1, wherein said current limiting eans includes firstand second transistor means coupled between the error signal and signalground and the error signal and a voltage source, respectively.
 9. Aloop filter for frequency synthesizing means, said frequencysynthesizing means including a signal source for generating a referencesignal, phase detecting means coupled to the reference signal and to afirst feedback signal for generating an error signal, voltage-controlledoscillating (VCO) means coupled to the loop filter, and dividing meanscoupled to the VCO means for providing the first feedback signal, saidloop filter comprising:current limiting means coupled to the errorsignal from the phase detecting means for generating a current-limitedoutput signal; filtering means coupled to receive the current limitingmeans output signal and including a series-coupled first resistive meansand capacitive means, for filtering the current limiting means outputsignal to provide a steering signal, the steering signal being coupledto the VCO means; amplifying means coupled to receive the output of thecapacitive means for generating a second feedback signal; and secondresistive means coupling the second feedback signal to the currentlimiting means output signal for substantially reducing the duration oftransients on the current limiting means output signal.
 10. The loopfilter according to claim 9, further including second filtering meansintercoupled between the first filtering means and the VCO means forfiltering the steering signal.
 11. The loop filter according to claim10, wherein said second filtering means comprises first and second phaseshifting means coupled in series between the first filtering means andthe VCO means for filtering and phase shifting the steering signal. 12.The loop filter according to claim 9, wherein said current limitingmeans includes first and second transistor means coupled between theerror signal and signal ground and the error signal and a voltagesource, respectively.